Composite gas sampling system

ABSTRACT

A sampling system for collecting periodic composite and/or non-composite samples of vaporized gas during a transfer process from a vaporizer of a cryogenic hydrocarbon liquid including 1) a direct sample pathway to a gas analyzer for instantaneous, real-time vaporized gas analysis, 2) a speed loop pathway for directly collecting fresh vaporized gas samples for subsequent analysis, and 3) a composite sample pathway including a pressurized sample accumulator for collecting a plurality periodically obtained samples of a select volume during the transfer process to create a composite sample of the vaporized gas.

FIELD OF INVENTION

This invention relates to collecting composite gas samples for analysis,and particularly for composite sample collection after a cryogenic gashas been conditioned by a system of the type described in U.S. Pat. No.7,484,404, Thompson, which is owned by the Applicant herein and sold asa Mustang Sampling system. More particularly, this composite samplesystem is complementary to a gas sample conditioning vaporizer employedto collect conditioned vaporized samples of liquid natural gas and/ornatural gas liquid.

BACKGROUND

Liquid natural gas sampling is governed by the standard ISO 8943 andGIIGNL LNG custody transfer handbook. Europe and other areas of theworld impose additional requirements such as mandating compositesampling of transferred LNG contents, and particularly, that from tankership off-loading. The standard calls for a composite sampling to becollected for the duration of time that a ship is off-loading. Thesampled gas stream is transferred to small cylinder sample containersfor storage and comparison to continuous online analysis averages.

During transfer processing of a liquid natural gas shipping vehicle, itis desirable to obtain accurate sampling for auditing the energy contentof the off-loaded LNG. This can be accomplished using known techniquessuch as periodic direct sampling from a takeoff vaporizer stream and/orcomposite sampling. While direct sampling allows for immediate analysisby an appropriate analyzer such as a gas chromatograph, it provides anaccurate portrayal of the content of the LNG off-loaded from the vehicleduring the entire transfer process only by extrapolating selectedaccumulated data. Additionally, manual direct sampling can be takenintermittently, for example, at ¼, ½, and ¾ of vessel cargo transfer.Automatic composite sampling is used to obtain particular volumes ofvaporized LNG at select periodic intervals during the transferprocessing. However, analysis of the typical composite sample content isavailable only after the transfer processing is complete.

Conventional composite sampling technology for LNG typically takes theform dome or floating piston systems. Dome systems are bladder based andrequire a fluid (typically water) to isolate the collection dome fromthe ambient environment and maintain pressure on the collected samples.The resulting extracted composite sample is subsequently transferredfrom the dome to sample cylinders for analysis and/or storage for laterqualitative analysis. Because dome systems rely on fluid/water failureof associated water seals will contaminate the composite sample.

Floating piston samplers are of a more simple construction than domesamplers and avoid the introduction of water/fluid as the seal method,but rely on mechanical seals. Correspondingly, floating piston systemsare believed to minimize the introduction of other ambient gases (e.g.,oxygen) into the composite natural gas sample. However, floating pistonsystems include a number movable parts and seals as well as requiring amotive source to pressurize the piston. Not only do such movable partsintroduce sources for sample contamination from leakage and the like,but also it is known that such systems employ relatively higherpressures to evacuate the sample chamber during cycling.

SUMMARY OF INVENTION

It is an object of the present invention to provide a novel samplesystem and methodology for conditioned vaporized, gas during transferprocessing.

It is another object of the present invention to overcome theaforementioned problems associated with conventional structures of theprior art.

Another object of the present invention is to provide an expedient toovercome recognized problems with existing sample collection techniques.

A further object of the present invention is to provide greaterflexibility in customizing the sampling technique selection andexecution for any particular transfer operation.

Still another object of the present invention is selectively providecoincidental fresh gas and composite gas sample sequestration foranalysis.

Yet another object of the invention is to provide a convenient,integrated composite gas sample system with a minimum of moving parts.

These and other objects are satisfied by an apparatus for capturing aperiodic gas sample following vaporizing and conditioning into a gasphase of a cryogenic liquid hydrocarbon source during transferprocessing, comprising: at least a first and a second vaporized sampleinput lines each incorporating at least a first direct feed line, asecond speed loop line, and a third accumulator feed line; each of thefirst direct feed lines of the first and second vaporized sample inputlines being directly connected to a gas analyzer for on-line, real-time,periodic analysis of a non-composite gas sample; each of the speed looplines being connected to a speed loop having an a pressure regulator, ahigh pressure pump, a plurality of solenoid controlled valves forcontrolled filling of a plurality of non-composite sample cylinders forstorage of fresh gas samples obtained at specified processing intervals,and a by-pass outlet to a boil-off-gas system; each of the accumulatorlines including at least one solenoid controlled valve, said accumulatorline for passing a gas sample of a select volume at select timeintervals to a pressure regulator, a solenoid controlled valve, and agas accumulator for receiving multiple periodic select volume gassamples to create a composite gas sample, and a high pressure pump tomaintain pressure sufficient in the gas accumulator to prevent dew pointdrop-out of the gas sample in the accumulator, a valve controlled outletfrom the gas accumulator, and a plurality of sample grab cylinders forreceiving composite samples from the gas accumulator upon completion ofsource processing.

Other objects are satisfied by a system for selected sampling cryogenicliquid hydrocarbon source where the liquid hydrocarbon has beenvaporized and conditioned by a vaporizer during transfer processing,comprising: a housing; a controller for controlling the gas samplingoperation contained within the housing; a vaporized gas port providing afirst, a second and a third gas stream feed lines adapted to receive avaporized gas sample of a select volume at a select time; a gas analyzerconnected to said first gas stream feed line; a speed loop connected tosaid second feed line; a plurality of removable, sample cylindersconnected to the speed loop for collection of non-composite freshsamples at select times directly from the vaporizer; an accumulatorconnected to said third gas stream for receiving a select volume of gasto create a composite sample of vaporized gas; a pump associated withsaid accumulator to maintain accumulator pressure at a level to preventdew point drop out of the vaporized gas; a plurality of removable samplegrab cylinders for receiving composite vaporized gas samples from theaccumulator; and a residual gas removal array for removing residual gasas from the system following transfer processing of the cryogenic liquidhydrocarbon.

Still other objects are satisfied by the method A method for sampling ofvaporized gas from a cryogenic liquid hydrocarbon liquid using a gassample system, comprising the steps of: obtaining a first vaporized gassample of selected volume and at first select intervals from a vaporizerconnected to a cryogenic liquid hydrocarbon repository; passing a selectvolume of said first vaporized gas sample to a first sample grabcylinder; pumping a second select volume of said vaporized first gassample to a composite sample accumulator tank under pressure sufficientto prevent dew point dropout; obtaining a second gas sample of selectedvolume at select interval different from the first select interval;passing a first select volume of said vaporized second gas sample to asecond sample grab cylinder; pumping a second select volume of saidvaporized second gas sample to the composite sample accumulator tankunder pressure sufficient to prevent dew point dropout to obtain acomposite gas sample; passing the composite gas sample to a select oneof a plurality of removable composite sample collection cylinders forreceiving said composite vaporized sample from the accumulator tank;removing the select one of the composite sample cylinders; and removingat least said first sample grab cylinder.

The sample system of the present invention is designed to take timedsamples after an sample conditioning system has converted a liquidsample to a gas from one or more input gas streams and provide anassociated gas chromatograph or other analyzer with a direct feed forinstant analysis, a fresh sample for subsequent analysis at select timeintervals during processing and a composite sample for subsequentanalysis representative of the entire gas content over the entire samplegathering process. That is, for example, after loading or unloading aship or container has been completed, the composite sample is in theaccumulator, the sampler is turned off. The associated sample cylinders,generally having a 500 cc volume are then filled from the accumulator.After the sample cylinders have been filled, the remaining gas in theaccumulator is vented and the system cleaned by use of gas purge, vacuumetc.

For control of the various processes carried out by the invention, useof a resident Programmable Logic Controller (PLC) is preferred. The PLCcontrols the sequential operations and timing of the composite samplepumps and gas line valves to perform the desired incremental sampling.Furthermore, when redundant or multiple gas streams and pumps areutilized, the invention contemplated that they all feed into a commonaccumulator rated for the system.

The inventive system does not rely only on volume measurements but alsofacilitates controlled, periodic sampling based on sample accumulationdiscrete, known time intervals. That is, the continuous sampling processto fill the sample cylinders provides more accurate results based on itsmore technical approach to sample collection by relying on select timingof sample accumulation rather than sample gas flow, alone. The inventioncontemplates a sample system that functions to obtain samples whilemaintaining relatively low pressures using a static structure (no movingparts) thereby minimizing risks of leakage and contamination.Furthermore, use of the invention substantially reduces the risk ofJoule-Thompson cooling and concomitant hydrocarbon dew pint dropoutwhich adversely impacts the accuracy of the samples being analyzed.

For definitional purposes and as used herein “connected” includesphysical, whether direct or indirect, affixed or adjustably mounted, asfor example, the composite sample system is connected to the avaporizer, the speed loop line is connected to the sample grab cylindersand the bypass. Thus, unless specified, “connected” is intended toembrace any operationally functional connection.

As used herein “transfer processing” means any processes involving themovement of cryogenic liquid from one place to another in theconventional sense as well as from or to any large cryogenic liquidnatural gas container such as ships, railcars or trucks.

As used herein “substantially,” “generally,” and other words of degreeare relative modifiers intended to indicate permissible variation fromthe characteristic so modified. It is not intended to be limited to theabsolute value or characteristic which it modifies but rather possessingmore of the physical or functional characteristic than its opposite, andpreferably, approaching or approximating such a physical or functionalcharacteristic.

In the following description, reference is made to the accompanyingdrawing, and which is shown by way of illustration to the specificembodiments in which the invention may be practiced. The followingillustrated embodiments are described in sufficient detail to enablethose skilled in the art to practice the invention. It is to beunderstood that other embodiments may be utilized and that structuralchanges based on presently known structural and/or functionalequivalents may be made without departing from the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a composite sample system in accordance with anembodiment of the invention.

FIG. 2 is a schematic diagram of an embodiment of the invention.

FIG. 3 is a schematic diagram of a multi-stream, multi-sample collectionembodiment of the invention.

FIG. 4 is a schematic diagram of an embodiment of the invention thatwith a bladderless accumulator and a vacuum unit for removing residualgas from the system upon completion of a transfer process.

DETAILED DESCRIPTION OF THE DRAWINGS

The embodiment illustrated in FIG. 1 includes the composite samplesystem 10 contained in housing 11. The system 10 provides arepresentative gas composition sample by taking small bite size samplesover a period of time. The system 10 includes a programmed logiccontroller (PLC) 12 with a connection to a remote communication facilityfor controlling the valves and solenoids and indicator lights for systemoperation as well as monitoring system status. In addition to thecontroller 12, the housing 11 of sampling system 10 contains samplepumps 14, a plurality of removable accumulator grab sample cylinders 16(four cylinders are used in the illustrated system), a plurality offresh sample cylinders 18 each for receiving a respective gas sample ata specific interval (e.g., ⅓ load transfer, ½ load transfer, and ⅔ loadtransfer) during the transfer processing. The grab cylinders 16 areconnected to accumulator 20 through appropriate tubing for communicationthe composite sample. The accumulator 20 which receives a multiplicityof samples of a specified small volume, e.g., 0.5 cc, at a preset timeinterval (i.e., 1 sec.) under conditions (pressure and temperature)maintained to prevent dew point dropout. Similarly when feeding thecomposite gas from the sample accumulator 20 to the respective grabsample cylinders, the associated pump(s) must maintain the pressure ofthe gas to prevent dew point drop out. For maintenance of systemstability, the interior of cabinet 11 includes the electrical housingheater 22 to maintained temperatures at an elevated level.

In operation, the composite sample system according to the inventiongrabs a fresh, 0.5 cc, sample every second, storing it in theaccumulator 20 for transfer to the associated grab cylinders 16following the conclusion of the transfer operation. The composite samplecan then be removed and transferred to a laboratory or analyzer site forsubsequent analysis.

The invention contemplates residual sampled gas removal at theconclusion offloading or transfer processing to reset the system for thenext processing operation. FIG. 2 illustrates a gas purge subsystembased on nitrogen gas fed into the system from the tank 24. The purgingoperation can be completely automated or may be semi-manual (thenitrogen regulator is opened manually when the PLC has been instructedto open the system valves for purging.

Turing to the embodiment illustrated in FIG. 3, it illustrates amultipath, multi-vaporizer takeoff system for multiple simultaneousinputs. This embodiment also provides speed loops 24 that providealternative feed lines for the vaporized gas directly from the vaporizerto one or more select collection cylinders and/or overflow from the gaspumps 14 associated with the accumulator 20. The particular arrangementprovides the system user with the greatest flexibility by allowing foreither collection cylinder set to be used for non-composite sampling aswell as providing venting to line feed 28 for a BOG (boil-off-gas)collector/header system.

Another option for resetting the system is illustrated in FIG. 4 where avacuum generating unit 26 is disposed in line with the system andactivated by the PLC 12 has all valves opened, to create a negativesystem pressure and move the residual gas to an appropriate outlet suchas a BOG header 28 or using the above-mentioned positive pressurepurging gas, e.g., Helium, from the tank 23 to push the residual gas toa vent or the like. Once removed, the system has been effectively resetand is ready for the next sampling operation.

The invention has been disclosed in the forgoing specification. It isunderstood by those skilled in the art that many modifications andembodiments of the invention will come to mind to which the inventionpertains, having benefit of the teaching presented in the foregoingdescription and associated drawings. It is therefore understood that theinvention is not limited to the specific embodiments disclosed herein,and that many modifications and other embodiments of the invention areintended to be included within the scope of the invention. Moreover,although specific terms are employed herein, they are used only ingeneric and descriptive sense, and not for the purposes of limiting thedescription invention.

We claim:
 1. An apparatus for capturing a periodic gas sample followingvaporizing and conditioning into a gas phase of a cryogenic liquidhydrocarbon source during transfer processing, comprising: at least afirst and a second vaporized sample input lines each incorporating atleast a first direct feed line, a second speed loop line, and a thirdaccumulator feed line; each of the first direct feed lines of the firstand second vaporized sample input lines being directly connected to agas analyzer for on-line, real-time, periodic analysis of anon-composite gas sample; each of the speed loop lines being connectedto a speed loop having a pressure regulator, at least one high pressurepump, a plurality of solenoid controlled valves for controlled fillingof a plurality of non-composite sample cylinders for storage of freshgas samples obtained at specified processing intervals, and a by-passoutlet to a boil-off-gas system; each of the accumulator lines includingat least one solenoid controlled valve, said accumulator line forpassing a gas sample of a select volume at select time intervals to apressure regulator, a solenoid controlled valve, and a gas accumulatorfor receiving multiple periodic select volume gas samples to create acomposite gas sample, and a valve controlled outlet from the gasaccumulator, where said at least one high pressure pump maintainspressure sufficient in the gas accumulator to prevent dew point drop-outof the gas sample, and a plurality of sample grab cylinders forreceiving composite samples from the gas accumulator upon completion ofsource processing.
 2. The apparatus of claim 1 further comprising acontrol unit for controlling the solenoid valves, flow operations, speedloop fresh non-composite sample collection timing, and accumulatorcomposite gas sample retention.
 3. The apparatus of claim 2 furthercomprising a housing cabinet including a heater and over pressure vent.4. The apparatus of claim 3 further comprising a separate analyzercabinet connected to or within said housing cabinet defining a housingcabinet for housing at least one gas analyzer.
 5. The apparatus of claim4 where, said analyzer is a gas chromatograph, and further includes aheater in the analyzer cabinet and a calibration gas station.
 6. Theapparatus of claim 1 comprising a third vaporized gas sample input lineand a second, duplicate accumulator line connected to a said gasaccumulator for simultaneous sampling from multiple inputs from thesource.
 7. The apparatus of claim 6 comprising a residual gas removalsystem following completion of processing sampling operations.
 8. Theapparatus of claim 7 where the residual gas removal system is a gaspurge system and where the Boil-off-gas system comprises a boil-off gasheader.
 9. The apparatus of claim 7 where the residual gas removalsystem includes an evacuation method.
 10. The apparatus of claim 1further comprising at least one flow controller disposed in said speedloop line and a second flow controller disposed in said accumulator linefor diverting the gas sample to said speed loop line, and where said atleast one high pressure pump comprises a first and a second highpressure pump, said second high pressure pump being disposed in saidaccumulator lines.
 11. The apparatus of claim 10 further comprising atleast a first second and third check valves to prevent backflowrespectively between said second high pressure pump and gas accumulator,said accumulator line and said speed loop line, and said sample grabcylinders and said boil-off gas system.
 12. The apparatus of claim 11further comprising a control unit for controlling the solenoid valves,flow operations, and sample introduction into said accumulator.
 13. Theapparatus of claim 12 where there are three non-composite samplecylinders for storage of fresh non-composite samples obtained at ⅓, ½and ⅔ of the elapsed processing intervals.
 14. The apparatus of claim 13where there are four removable, receiving composite sample grabcylinders.